This invention is directed generally to improvements in graphics display apparatus and specifically to cathode ray tube (CRT) display apparatus having touch panels for controlling the display of graphics responsive to a user touching selected areas on a CRT faceplate.
Conventional graphics display apparatus of the type under consideration include a CRT having a faceplate surrounded by a plurality of energy sources and detectors forming a network or grid of crossing energy paths closely adjacent to the viewing surface of the faceplate. Any interruption of crossing energy paths, for example by a user touching the faceplate, results in a signal to a CRT graphics controller for changing the graphics displayed in accordance with the area touched and the system software program. Thus the level of user skill and degree of user knowledge required for effective interaction with a computer terminal are greatly reduced to a simple ability to read and to touch. For obvious reasons such systems are very attractive and greatly enhance the marketability of computers.
A number of touch panel display system for CRTs are in existence. They generally comprise a plurality of light sources (usually light emitting diodes, i.e., LEDs) for generating light energy in the infrared range and suitable receptors for receiving such energy. The sources and receptors are usually positioned along the horizontal and vertical axes of the CRT faceplate. The prior art discloses a number of sophisticated systems for assuring the integrity of the energy paths and for delineating one path from another. The art also discloses well-known techniques for sequentially energizing the energy sources, deriving electrical signals representative of the intensity of the energy in each path and comparing the intensity of that energy to a predetermined threshold energy level. This process of interrogating the energy paths determines whether the paths have been interrupted. The predetermined threshold energy level is made sufficiently low to embrace the full range of energy levels in the paths, which vary significantly depending upon source characteristics, transmission media, accuracy of receptor alignment, and the like. While a significant improvement in minimizing the effects of ambient light on the touch display system has been effected with the use of bandpass amplifiers for restricting response to the energy wavelength of the source used, many problems remain with respect to positively determining whether an energy path has been interrupted or not.